The present invention is directed to communication systems, and is particularly directed to a system for automatically locating and thereby facilitating removal of energy reflection anomalies, such as bridged taps, and the like, that may impair digital communications along a wireline telecommunication link.
In the face of the increasing demand for a variety of to digital communication services (such as, but not limited to internet services), telecommunication service providers are continually seeking ways to optimize the bandwidth and digital signal transport distance of their very substantial existing copper plant, that was originally installed for the purpose of carrying nothing more than conventional analog (plain old telephone service or POTS) signals.
In addition to the inherent bandwidth limitations of the (twisted pair) copper wire medium, service providers must deal with the fact that in-place metallic cable plants, such as that shown at 10 in the reduced complexity network diagram of FIG. 1, linking a central office 12 with a subscriber site 14, typically contain one or more anomalies, such as but not limited to load coils (used to enhance the wireline""s three to four kilohertz voice response), and bridged taps 16, to which unterminated (and therefore reflective) lateral twisted pairs 18 of varying lengths may be connected.
Because these discontinuities cause a portion of the energy propagating along the wireline link to be reflected back in the direction of the source, at the high frequencies used for digital data communications (e.g., on the order of one MHz), such reflections can cause a significant reduction in signal amplitude, when (counterphase) combined with the original signal, thereby disrupting digital data service. In order to locate these reflection points, it has been conventional practice to employ interactive, time domain reflectometry (TDR), which relies upon the ability of a skilled technician to make a visual interpretation of a displayed TDR waveform, and thereby hopefully identify the bridged taps, and the lengths of any laterals that may extend therefrom. Because this process is subjective, it is not only imprecise, but is very difficult to automate.
In accordance with the present invention, shortcomings of conventional TDR-based schemes for locating energy reflecting anomalies, such as bridged taps and the like, along a wireline telecommunication link, are effectively obviated by means of an objective, frequency domain reflectometry (FDR)-based mechanism. The test mechanism of the invention may be implemented in a processor-controlled test head installed in a central office, or as part of test signal generation and processing circuitry of a portable craftsperson""s test set.
The test head contains test signal generation and processing circuitry, that is operative to execute a frequency domain reflectometry (FDR) algorithm, through which the line is stimulated by means of a linearly swept (stepped) sinusoidal waveform, to invoke a line response that is readily measured and analyzed to reveal the locations (distance from the source) of any impedance mismatch reflection discontinuities (e.g., bridged taps and the like).
A control processor is programmed to generate a sinusoidal waveform that is linearly stepped from a minimum frequency of 0 Hz (DC) to a maximum frequency fmax. As the frequency of the test waveform applied to the line is swept, the signal level at a line access point is monitored via an input amplifier, digitized, and then and stored in a signal measurement buffer. The amplitude of the measured signal response will exhibit a variation with frequency that is a composite of fluctuations in impedance due to any reflection points (e.g., bridged taps) along the line.
Once captured, the response data is weighted to optimize the accuracy of the analysis. Any DC level is removed, and a window, such as a Hanning window, is used to remove discontinuities between start and end values of the captured data set, to avoid spurious results. A loss compensation function is applied to the modified data set, to compensate the frequency response characteristic of the line for loss over distance and frequency. A frequency-dependent propagation constant is derived in terms of the resistance, inductance, capacitance and conductance of the line per unit length. The real part of the propagation constant is the attenuation along the line per unit length. The attenuation of the envelope of a signal propagating along the line is an exponential function of the propagation constant. The effect on the frequency response waveform is that amplitude decay is less pronounced for reflected signals propagating on shorter loops, since the shorter distance offsets the effects of the loss at high frequencies. Because the length of the line under test is unknown, a compromise between the two extremes provides compensation for the overall frequency response waveform irrespective of distance.
The loss-compensated data is then processed by a frequency analysis operator, such as discrete Fourier transform (DFT), which decomposes the composite line signal response into frequency bins associated with the individual reflectors"" frequency fluctuations. A threshold is established for the contents of the frequency bin data produced by the DFT, in order to distinguish between useful or significant energy and spurious energy. A frequency bin is considered to contain significant energy, if its contents exceed the threshold for that bin number. Any frequency bin whose contents exceed its threshold are subjected to frequency domain reflectometry (FDR) analysis.
In the context of detecting bridge taps along a wireline telecommunication link, a waveform propagating downstream along the wireline combines with a waveform reflected from a bridged tap and returning upstream along that wireline. Since the downstream and upstream propagating waveform components have the same frequency, the composite waveform will have a local minimum due to destructive interference at some time delay when the arguments of the two waveform components differ by xcfx80 radians. Nulls will occur for other frequencies, where the arguments of the waveform components differ by odd multiples of xcfx80. A linear sweep of a wireline having a single reflection point (e.g., bridged tap) will produce nulls at frequencies fo, 3fo, 5fo, 7o, etc. In general, the null repetition rate in the frequency domain Fn may be given by: Fn=1/2fn, where fn is the lowest frequency at which a null occurs when the delay t=tn. Fo corresponds to to and, in general, Fn corresponds to tn and is the same as the round-trip delay of the signal from the line access location to the point of reflection along the line and back.
In order to determine the length of time required for the waveform to propagate to the impedance-mismatch reflection point, it may be observed that to is representative of the total time required for the downstream propagating waveform to be reflected back to the access location at which the measurement is taken. This one-way delay ti is equal to to. To determine the distance of this reflection point from the access location, the propagation velocity vp of the waveform along the wireline, which is readily calculated, is employed. The distance from the access location to the location of the impedance mismatch reflection is inversely proportional to frequency, and the minimum resolvable distance Dmin=vp/2fmax.
Where the line under test contains plural discontinuities, the response waveform seen at the signal measurement point will contain multiple components produced by the plurality of reflection points. Since these reflected waveforms components are generally associated with impedance discontinuities caused by physical characteristics in the wireline separated by varying distances from the source, the delays associated with these reflections will be mutually different, so that their frequencies will be mutually different. As each delay produces its own unique frequency, then by identifying the various frequencies, the two-way delay times of a reflection from a wireline discontinuity may be readily determined, so that the distance to the impedance discontinuity may be determined.
To determine the individual values of two-way delay time, the frequency response waveform produced by stimulating the wireline under test with a linearly swept sinusoidal waveform is sampled at discrete frequency steps of (fmax/N). The DFT produced will yield values that area proportional to the magnitudes of the various null repetition rates Fk. The contents of the first frequency bin are the DC component of the swept response, while a respective bin m contains the magnitude of the null repetition rate (mxe2x88x921)Fo, for m=2,3,4, . . . N/2. Namely, the various energy bins of the response represent energy associated with the time delays to, 2to, 3to, etc., and contain the magnitude of the waveforms delayed by (mxe2x88x921) to for m=2,3,4, . . . N/2.
As a result, the bins of the DFT, which represent different round trip delay times of the swept waveform, can be employed to determine the distances from the access location to energy-reflecting anomalies. The distance to a reflection point may be determined by multiplying the one-way delay by the velocity of propagation of the swept waveform. In general, the bins of the response represent distances that are integral multiples of the delay to. Namely, the distance Dmxe2x88x921 associated with a bin mxe2x88x921 is equal to (mxe2x88x921)tovp/2 or [(mxe2x88x921)to]vp/2 for m=2,3,4, . . . N/2. Thus, there is a one-to-one correspondence between the bins of DFT and distances to the reflection points along the wireline.